A variety of elastomeric mounts are used to attach components to an automobile, e.g., engine mounts, transmission mounts, and chassis mounts. Ideally, the mount must be rigid enough to support the weight of the component, but should be designed so that the vibrations from the powertrain, wheels, or other sources are minimally transmitted to the driver and passengers. These goals have led to a strategy whereby the mount itself is often designed to possess a mechanical resonance at a frequency below that of the component to be supported. Drawbacks of this approach include that a different mount must be designed for each mount location, that the mount cannot be re-tuned to accommodate options or accessories that alter the component's resonant frequencies, and that the mount cannot be adapted to the various vibrational resonances that are encountered under different operating conditions.
One approach that allows the adaptation to varying conditions is to introduce a variable element in the mount. An example is an engine mount possessing two fluid-filled chambers connected by an orifice. By mechanically varying the orifice size (e.g., by a valve driven by engine vacuum) one can vary the stiffness and damping of the mount. This is in fact the approach taken in one production engine mount.
Variable-stiffness mounts using electrorheological (ER) fluids use a simple variation on the above scheme. ER fluids are dense suspensions of small, polarizable particles in viscous, insulating liquids. When high electric fields are applied to these slurries, their apparent viscosities increase dramatically. Following the above approach, an ER mount can be constructed by filling two chambers with ER fluid and connecting them with an orifice. By applying a high electric field at the orifice, e.g., by using two parallel-plate electrodes, the flow of the ER fluid can be controlled and the stiffness of the mount can thereby be adjusted. Because the rheological response of most ER fluids to changes in applied field is fast, semi-active damping control is possible.
Several drawbacks are associated with the use of ER fluids. Since they are fluids, they must be sealed against leakage. Because the suspended particles generally have different density than the suspending liquid, the particles tend to eventually settle out of suspension due to gravity. One way to avoid these drawbacks is to suspend the polarizable particles not in a fluid, but rather in a viscoelastic solid. The stiffness of this ER solid can be varied by an applied voltage or electric field. These ER solids could be incorporated into a variety of elastomeric mount designs. It is conceivable that a single mount design would be utilized in any number of different applications simply by changing the operating voltage of the mount design to achieve different stiffnesses. Multiple benefits would presumably accrue from such a strategy: inventories could be reduced; new components could be accommodated with out redesigning the mount; accessories and options (e.g., air conditioning, turbochargers, etc. on an engine) that would affect the component resonance would not necessitate a mount redesign.
One ER solid is taught in U.S. Pat. No. 5,364,565 issued Nov. 15, 1994 to Li et al., and titled "Electro-viscoelastic Gel-Like Solids", which is commonly assigned with this invention. That material is a partially crosslinked solid made from liquid polymer precursors some of which are not as stiff as that of the present invention and hence, without the application of an electric field, not optimally useful as, e.g., engine mounts. Advantageously, the present invention elastomer has the necessary stiffness and durability to be used in manufacturing engine mounts and, because it comprises natural rubber as the host material, the elastomers could be easily mass-produced using production techniques well-established in the rubber industry.